Process for the oxidation of alkanol to aldehyde using supported catalyst



Aug. 26, 1958 F. J. SHELTON ET AL PROCESS FOR THE OXIDATION OF ALKANOLTO ALDEHYDE USING SUPPORTED CATALYST Original Filed Aug. 27. 1954 2Sheets-Sheet 1 ATTORNEY PROCESS FOR THE OXIDATION OF ALKANOL TO ALDEHYDEUSING SUPPORTED CATALYST Frederic J. Shelton and Eugene M. Barrentine,Seattle, Wash, assignors to Reichhold Chemicals, Inc, Betroit, Mich.

Original application August 27, 1954, Serial No. 452,685, now Patent No.2,812,308, dated November 5, 1957. and this application May 24, 1957,Serial No.

2 Claims. (Cl. 260 603) The invention relates to a process for theproduction of aldehydes by catalytic oxidation of alcohols, to animproved active catalyst for use in such process, to the method offorming such catalyst from an inactive precursor, and especially to theinactive catalyst precursor and to the method of producing the same.

The present application is a division of our application Serial No.452,685, filed August 27, 1954, now U. S. Patent No. 2,812,308.

More particularly this invention relates to an improved iron molybdenumoxide catalyst precursor particularly suitable for the manufacture of anactive catalyst useful in the production of aldehydes such asformaldehyde, acetaldehyde, butyraldehyde and the like, by oxidation inthe vapor phase using the corresponding alcohols.

Our active catalyst is also useful for the production of substitutedaldehydes such as di-aldehydes from glycols, glyoxylic acid fromglycollic acid, and the like.

The catalyst precursor is not in itself suitable for direct use as a.catalyst for the oxidation of alcohols to aldehydes, but becomes a veryeificient catalyst when converted to active catalyst Within the alcoholconverter. In our process an alcohol is vaporized, mixed with air, andpassed through a converter containing catalyst packed tubes heated to asuitable conversion temperature. The processed gases are then cooled torecover the products. Heretofore it has been necessary to prepare acatalyst at the site of use in order to cut down on handling of the moreor less fragile catalyst material. For the first time, in practicing ourinvention, it is possible to prepare supported catalyst precursor in onecity and ship the rugged catalyst precursor to a plant located inanother city for final installation and conversion to an active catalystin an aldehyde producing plant.

Our catalyst dilfers from previous iron oxide-molybdenum oxide catalystin being supported on a suitable nonmetallic carrier, preferablysintered silicon carbide. We have investigated many catalyst carriersupports, such as steel ball bearings, steel nuts, sintered aluminumoxide, pelletized silicon dioxide, crystalline silicon carbide, sinteredsilicon carbide, crystalline tungsten carbide, sintered iron carbide andsintered iron oxide. Of these only the carbides and sintered iron oxideare satisfactory, silicon carbides being preferred, and the best form ofsilicon carbide is the sintered material. A particularly good grade ofsintered silicon carbide is that manufactured by the Carborundum Companyand is designated as Carborundum brand, poly surface pellets, siliconcarbide, type CMM, 6 to 10 mesh.

In preparing our catalyst we prefer to first make up a solutioncontaining a soluble iron salt and a soluble molybdenum salt. Thissolution is then added to the sintered silicon carbide carrier and themixture dried at atmospheric boiling temperature, stirring continuouslyduring the drying period. It is important that the supported catalyst bedried adequately for ease in handling but it should not be taken tocomplete dryness since that produces a fragile catalyst. We have foundthat small amounts of water and ammonia should be present in thecatalyst precursor in order to have a rugged catalyst capable ofshipment by common carrier. We prefer a final moisture content in thecatalyst precursor or between 2 and 25% by weight.

While we prefer to use a completely water soluble solution of iron andmolybednum it is possible to precipitate an iron-oxide molybdenum oxidemixture in the presence of the silicon carbide carrier and dry themixture in conventional fashion. It is preferable to carry out thedrying in a drier equipped with some scraping action. A rotary tunneldrier is preferred. Drying on trays with no agitation is notsatisfactory due to partial separation of the iron-molybdenum oxidesfrom the silicon carbide.

During drying the precipitate, if not agitated, tends to separate fromthe carrier due to particle size difference. In one stage during dryingthe precipitate passes through a sticky or gummy stage which forms aseparate layer on the container holding the precipitate and the carrier.This layer impedes the transfer of heat, lengthening out the drying timeand giving more chance for separation of the precipitate from thecarrier. Agitation is desirable to keep the precipitate spread uniformlyover the surface of the carrier during the critical sticky drying stage,thus preventing agglomeration of coated carrier particles.

While we prefer to use ferric oxalate or ferric ammonium oxalate we findthat ferric citrate, ferric ammonium citrate, ferric nitrate, ferricchloride, or any other ferric or ferrous salt having a volatile ordecomposable anion is satisfactory.

Ammonium molybdate is the preferred molybdenum containing compound.However, any molybdenum containing salt having a volatile ordecomposable anion or cation in combination with the molybednum may beused.

The following is a preferred example of a high iron containing catalystprecursor:

Example I A coated carrier catalyst precursor was prepared as follows:

(1) 45.0 pounds of Baker-s reagent grade ammonium heptamolybdate (NH MoO.4H O were dissolved in pounds of distilled water and labeled solutionA.

(2) 33.0 pounds of Bakers reagent grade ferric ammonium oxalate Fe(lll-I(C O .4H O were dissolved in 60 pounds of distilled water and labeledsolution B.

(3) '125 pounds of Carborundum brand, poly surface pellets, siliconcarbide, type CMM, 8+10 mesh were added to solution B in the drier.

(4) Solution A was added to solution B-carrier slurry with agitation.

(5) The resulting mixture was then boil dried to a moisture content ofabout 16%, with constant agitation in a steam heated mechanical drier atatmospheric pressure.

('6) The coated carrier was then dried at 220 F. for 16 hours thenplaced in the converter and the final drying and activation done withthe catalyst in place.

This produces a coated carrier precursor of about 35% greater weightthan the weight of the carrier used. Heating this precursor to 700 F.produces a 13.5% loss of volatile material. The dried coating contains aweight ratio of Fe O /MoO of about 1:5.03 as determined by chemicalanalysis.

The final conditioning and activation of the catalyst was brought aboutby loading the catalyst into the converter and slowly increasing theconverter bath temperature to about 495510 F. in 2 hours and at the sametime blowing a gentle stream of air (space velocity 50 per minute-notcritical) through the catalyst bed. This blowing of air through thecatalyst bed was continued at the elevated temperature until there wasno further trace of ammonia or water in the exhaust gas from theconverter. At this time the methanol feed was started.

With a converter tube inside diameter containing a concentric outsidediameter thermowell, packed with the above catalyst to a height of 24.5inches, operating at a bath temperature of 722 F. and feeding a mixturecontaining 1.20 cubic feet per minute of air (calculated to standardconditions of 32 F. and 1 atmosphere) and 4.42 cc./minute of methanol(about 8% methanol by weight in the feed) heated to about 350 F. beforeenterng the catalyst bed a yield of 87.2 pounds of formaldehyde gas wasobtained for every 100 pounds of methanol fed. The hot zone temperatureranged from 795 F. to 840 F.

Example 11 The following is an example of a low iron containing catalystprecursor:

(1) 45.0 pounds of C. P. grade ammonium heptamolybdate (NH MqO .4H Owere dissolved in 100 pounds of warm distilled water in a glass linedcontainer.

(2) 20.0 pounds of C. P. grade ferric ammonium oxalate Fe(NH (C O .4I-IO were dissolved in the ammonium heptamolybdate solution of step 1 byheating gently to about 140 F.

(3) The solution from step 2 was added to 125 pounds of 6 to 8 meshCarborundum brand, poly surface pellets, silicon carbide, type CMM inthe drier.

(4) The resulting mixture was then dried at atmosperic pressure to amoisture content of about 14% with constant agitation in a steam heatedmechanical drier.

' The coated carrier was then dried for 24 hours at about 220 F.

The catalyst coating produced on the silicon carbide carrier by thismethod contains a weight ratio of Fe O /MoO of approximately 1:10.25.

This catalyst was then packed in fiber drums and made ready for shipmentto the operating formaldehyde plants around the country.

The final conditioning and activation of the catalyst was brought aboutby loading the catalyst into the converter and slowly increasing theconverter bath temperature from about 70 F. to about 495510 F. in 4hours and blowing a gentle stream of air (space velocity 20 per minute)through the catalyst bed after a temperature of 200 F. was reached. Thisblowing of air through the catalyst bed was continued until all tracesof ammonium and water in the exhaust gas from the converter haddisappeared. The catalyst was then ready for operation and the methanolfeed was started.

With a converter tube of 1 inch inside diameter containing a inch 0. D.thermowell packed with the above catalyst to a height of 23 inches,operating at a bath temperature of 500 F. and feeding a methanol airmixture containing by weight of methanol at an air flow of 0.3 cubicfoot per minute (STP) and heated to about 390 F. before entering thecatalyst bed, a hot zone temperature of from 570 to 595 F. was generatedand a yield of 86.2 pounds of formaldehyde gas was obtained for every100 pounds of methanol fed.

Example III A catalyst precursor was prepared as follows: (1) 45.0pounds of ammonium heptamolybdate were dissolved according to theinstructions for Example II and the catalyst was prepared following theprocedure for verter of Example II and the methanol feed was started.With a methanol-air mixture containing 16% by weight of methanol at anair flow of 1.1 cubic feet per minute (STP) per 1 inch inside diametercatalyst tube and heated to 390 F. before entering the catalyst bed, ahot zone temperature of from 695 to 724 F. was observed and a yield of75 pounds of formaldehyde gas was obtained for every pounds of methanolfed.

In Allyn et al. Patent No. 2,812,309 an unsupported ironoxide-molybdenum oxide catalyst is disclosed. There are a number ofimportant dilferences between the catalyst of this application and theinvention of the present application. The most important difference isthe economic advantage arising from the surprising fact that oursupported catalyst may be used to oxidize methanol in the range from 0to 16% by weight of methanol in the methanol-air mixture where as thehighest concentration of methanol to air suggested in Patent No.2,812,309 is 11% by weight of methanol in the methanolair mixture. Thismeans that if our catalyst is introduced into a plant which has beenusing the unsupported catalyst Patent No. 2,812,309, the productivecapacity of that plant may be increased by over 45% without anycomplicating changes in operation. This is an important economicadvantage since it permits increased production without paying forincreased converter capacity. In using our catalyst the capital investedin plant facilities per pound of formaldehyde produced takes asignificant drop.

If in the operation of a methanol converter using an unsupportedcatalyst, a methanol-air mixture is used where the methanol content isabove 11% the exothermic oxidation reaction which takes place within thecatalyst tubes takes off explosively. This results in loss of catalystby blowing it out of the tubes and also results in more or less damageto the equipment. With our catalyst it is possible -to operate at amethanol content in the vapor of well over 11% with little danger ofexplosion. Only with more than 16% methanol in the feed does the dangerof explosion arise. The extra safety in using our catalyst alone issufficient reason for the commercial success of our catalyst.

We believe that the reason that our catalyst may be used to oxidizemethanol-air mixtures containing from 11- 16% by weight of methanolwithout danger of an explosion is because of the fact that lower hotzone temperatures and broader hot zones in the catalyst are generatedthan when the solid catalyst of Patent No. 2,812,309 is used. Acomparison of the observed temperatures along the length of a reactiontube containing our catalyst and a reaction tube containing theunsupported catalyst of Patent No. 2,812,309 operating under the sameconditions is shown in Fig. 1. It may be seen that the maximumtemperature in the tube containing our supported catalyst was lower andthe reaction zone as indicated by temperatures above 525 F. was longerthan that for the unsupported catalyst.

In a given formaldehyde plant the use of supported catalyst has animportant advantage over unsupported catalyst in that it costs less tofill the catalyst tubes in the converter with supported catalyst thanunsupported catalyst. Using comparable raw materials an unsupportedcatalyst of Patent No. 2,812,309 will cost about $742 per cubic footcompared to a cost of $156 per cubic foot for the catalyst supported onsilicon carbide. The saving in cost of catalyst is decidedly in favor ofthe supported catalyst.

The silicon carbide has considerable mechanical strength whichcontributes largely to the enhanced mechanical strength of the supportedcatalyst. At the end of the useful life of the catalyst it is found incommercial practice that the level of the catalyst in the tubes has notdecreased appreciably. This is evidence of the superior strength of thesupported catalyst over the unsupported catalyst which usually losesabout 10% volume under comparable conditions.

We have discovered that a most etficient active catalyst may be made byconverting the supported catalyst precursor within the converter by aheating and blowing process. In carrying out this conversion we placethe supported catalyst precursor in the catalyst tubes (usually to adepth of about 21" in a commercial converter having multiple 1" tubesarranged in parallel) and the bath surrounding the catalyst tubes isheated to a temperature between 200 C. and 300 C. and air containing noalcohol is blown through at a space velocity of about 20-80 per minute.The space velocity is not critical but should be sufiicient to removeany volatile contaminants as they are formed. Heating and blowing arecontinued until no ammonia (if present) or water comes off. After thispoint is reached the active catalyst is ready for operation and agaseous mixture of alcohol and air may be caused to flow through the hotconverter to give the aldehyde.

Other methods of dehydration and ammonia removal may be employed. Forexample, air may be dehydrated by chemical or physical means usingphosphorus pentoxide, calcium chloride, silica gel, or the like, and theresulting air passed through the catalyst at a time and temperaturesuflicient to remove all the Water and volatiles.

The surface character of the silicon carbide is important. We have foundthat crystalline silicon carbide is inferior to the more porous sinteredforms of silicon carbide. We have discovered that porous sinteredsilicon carbide having an apparent specific gravity of between 2.4 and2.9 is most satisfactory as a carrier material. Depending on conditionsand engineering de- 0 and 450 per minute with the optimum at about aspace velocity of 300 per minute.

A schematic flowsheet for the process for the production of formaldehydefrom methanol is shown in Fig. 2. In this process methanol is pumpedfrom a storage tank 10 to a steam heated vaporizer 11 and the methanolvapor and air are metered by means of standard orifice plate gas flowmeters into the venturi mixing chamber 12 and the hot mixed methanol-airstream is fed through the catalyst bed contained in a series of 1 inch0. D. boiler tubes four feet long inside the converter 13. A Dowthermheat exchange medium 14 surrounds the catalyst tubes of the converter.The reacted gases from the catalyst bed pass through a heat exchanger 15preheating the entering air. The cooled formaldehyde containing gas isthen run through an absorption tower 16 and the formaldehyde is absorbedin water to yield directly a commercial 37% formaldehyde solutioncontaining less than 1% methanol and 0.02% formic acid.

The invention has been described in detail for the purpose ofillustration but it will be obvious that numerous modifications andvariations may be resorted to without departing from the spirit of theinvention.

We claim:

1. A process for making aldehyde which comprises passing an alkanol andan oxygen containing gas through a converter at superatmospherictemperature in the presence of a supported active catalyst formed withinthe converter by heating and air blowing an inactive supported catalystprecursor produced by forming in situ an iron molybdenum oxalate coatingupon a carrier sign of the converter the mesh size of the siliconcarbide selected from a group consisting of carbides and sintered ironoxide and-partially drying said coating to a moisture content of from 2to 25% by weight of the coating,- the partially dried coating having amolar ratio of M00 to Fe O of between 3.4 and 11.1, the air blowing ofthe catalyst precursor being continued until substantially all traces ofmoisture and any remaining gas are removed prior to the introduction ofthe alkanol into the converter.

2. A process as set forth in claim 1, wherein the aldehyde isformaldehyde and the alkanol is methanol.

No references cited.

1. A PROCESS FOR MAKING ALDEHYDE WHICH COMPRISES PASSING AN ALKANOL ANDAN OXYGEN CONTAINING GAS THROUGH A CONVERTER AT SUPERATMOSPHERICTEMPERATURE IN THE PRESENCE OF A SUPPORTED ACTIVE CATALYST FORMED WITHINTHE CONVERTER BY HEATING AND AIR BLOWING AN INACTIVE SUPPORTED CATALYSTPRECURSOR PRODUCED BY FORMING IN SITU AN IRON MOLYBDENUM OXALATE COATINGUPON A CARRIER SELECTED FROM A GROUP CONSISTING OF CARBIDES AND SINTEREDIRON OXIDE AND PARTIALLY DRYING SAID COATING TO A MOISTURE CONTENT OFFROM 2 TO 25% BY WEIGHT OF THE COATING, THE PARTIALLY DRIED COATINGHAVING A MOLAR RATIO OF MOO3 TO FE2O3 OF BETWEEN 3.4 AND 11.1, THE AIRBLOWING OF THE CATALYST PRECURSOR BEING CONTINED UNTIL SUBSTANTIALLY ALLTRACES OF MOISTURE AND ANY REMAINING GAS ARE REMOVED PRIOR TO THEINTRODUCTION OF THE ALKANOL INTO THE CONVERTER.